Paint Technology and Tests Part 23

=Test No. 20, Special.= Primed and second coated with a green paint containing zinc oxide and barytes, ground in an oil having a low saponification value. Very slow drying was shown. Excellent results. No failure of color. Extremely glossy, waterproof surface presented.

CHAPTER XVI

STRUCTURAL STEEL PAINT TESTS

=The Necessity of Protective Coatings.= Most painters have in the past considered of minor importance the painting of iron and steel; any paint that would properly hide the surface of the metal being accepted without much question. The demand, however, for structural steel for office buildings, factories, steel cars, railroad equipment, etc., has doubled the output of structural paints, and created a demand for painters having a knowledge of the proper materials to use in the painting of steel, so that its life may be preserved, and its strength maintained.

Such knowledge is as important to the painter as a knowledge of how to properly select materials for the painting of wood, and how to temper these materials to suit the various conditions met with.

=The Cause of Rust.= Everyone is familiar with the appearance of rust, but few actually understand what causes rust. No attempt will be made here to present even an outline of the many theories advanced to explain the phenomenon of the rusting of iron, for the subject is as diverse as it is interesting. A brief resume, however, will be given of the now generally accepted theory that explains the subject. This theory is called the electrolytic theory. "Auto-electrolysis" is the term used to define the peculiar tendency of iron to be transformed from a metal possessing a hard l.u.s.trous surface, high tensile strength, and other useful properties, to a crumbling oxide that falls to the ground and again becomes part of the earth from which it was originally taken by man.

[Ill.u.s.tration: A Side View of Steel Test Fences]

This "going back to nature" is more readily accomplished by most of the steel produced to-day than by the old hand-made irons produced many years ago. It seems to be a curious fact that the more quickly a product or an article is fashioned by man, the more quickly it tends to return again to its original oxidized condition. Some manufacturers of steel, however, through an understanding of the causes of rust, have progressed in the manufacture of slow rusting materials, either by the elimination, or by the proper distribution of impurities.

When iron is brought into contact with moisture, currents of electricity flow over the surface of the iron between points that are relatively pure and points that contain impurities. These currents stimulate the natural tendency of the iron to go into solution, and the solution proceeds with vigor at the positive points. The air which the water contains oxidizes the iron which has gone into solution, and precipitates the familiar brown iron rust. Thus water, which acts as an acid, and air, which acts as an oxidizer, have combined together to accomplish the downfall of the metal.

[Ill.u.s.tration: Three Photomicrographs of Corroding Steel]

=Inhibition and Stimulation of Rust.= It is obvious that if means could be devised to stop the solution pressure of iron and make it resistant to the flow of surface electric currents, rust could be prevented. Such methods have been devised, and to better ill.u.s.trate how they operate, an a.n.a.logy may be drawn between iron in water and sh.e.l.lac in alcohol.

It is common knowledge that when sh.e.l.lac is placed in alcohol, the sh.e.l.lac will force itself into solution in the alcohol, and form a clear, transparent lacquer. If, however, there should be mixed with the alcohol a quant.i.ty of water, it would be found that the sh.e.l.lac could no longer go into solution, and it would remain in its original condition.

In the same way, if there be placed in water a small quant.i.ty of material, such as soluble chromates, or an alkaline substance like caustic soda or lime, it will be found that iron will no longer have a tendency to go into solution in this treated water, but will stay bright and clean. These materials which prevent the rusting of iron have been called by Cushman, who first advanced these explanations, "rust inhibitors," or materials which inhibit rusting. The paint maker, realizing the importance of these rust inhibitors, is incorporating them into paints designed for the protection of iron and steel, and the success which paints of this type have met with from a practical standpoint is a justification of what was first called the "electrolytic theory," which suggested their use.

By placing small, brightly polished steel plates into a mush of paint pigment and water, a determination may be made of the pigment's effect upon the metal. Some pigments, under such conditions, cause rapid corrosion of the steel plates. Such pigments are stimulators of corrosion, on account of acid impurities which they contain, or because of their effect in stimulating galvanic currents. Many carbonaceous pigments are of this type. Other pigments have the effect of keeping bright the steel plates and preventing rust. Such pigments are of the inhibitive type, and their action is to check or r.e.t.a.r.d the solution pressure of the iron.

=The Effects of Moisture.= It might occur to the reader that although paint pigments, when mixed up with water and brought into contact with the surface of steel, might show either an inhibitive or stimulative action, that it is by no means certain that the same tendency will be exhibited by pigments when they are properly mixed with linseed oil and laid out as a film upon the surface of steel. In answer to this, it may be well to state that almost no material used by mankind is absolutely dry. Linseed oil, as it is pressed from the seed, comes from the cells, carrying with it a certain small definite percentage of water, and it is quite certain that even the best linseed oil that goes into use is not theoretically dry. Everyone knows, of course, that oil and water do not readily mix and are, in fact, more or less repellent to each other. It is, however, true that, in spite of this, oils can carry quite a percentage of water, without the admixture being apparent to the eye. In addition to this, careful experiments have proved very conclusively that linseed oil films, even after they have oxidized and hardened, have the power to a certain extent of absorbing water from the atmosphere. It is, therefore, safe to say that no linseed oil film in a paint coating is dry all the time. As a matter of fact, there is abundant evidence to show that in rainy weather, and, in fact, when the humidity in the air is high, paint films have absorbed water. As the sun comes out and warms the paint coating, and the humidity content of the atmosphere falls, this water to a large extent evaporates out of the film, only to be taken up again when the weather conditions change. This action may be likened to a breathing of the paint film, that is to say, an indrawing of water under humid conditions, followed by an exhaling of water under dry conditions. With these facts in mind, it must be apparent that pigments laid out in intimate contact with the surface of steel are subjected at all times either more or less to the reactions produced by water contact. Furthermore, as it is a property of water to become saturated with the gases of the atmosphere, such as oxygen, carbonic and sulphurous acids, and other impurities, there is present in a protective paint film at all times the elements necessary to carry on the corrosive process and reactions.

An outline of Cushman's original research work, upon which has been based the cla.s.sification of pigments as inhibitors, stimulators, and inerts, is clearly presented in his report[38] as Chairman of Committee U of the American Society for Testing Materials, of which the following is an excerpt:

[38] Page 73, 1910 Proceedings of the American Society for Testing Materials.

[Ill.u.s.tration: Ferroxyl Tests on Painted Steel Surfaces. Upper Row Painted with Stimulative Paints--Lower Row with Inhibitive Paints.]

[Ill.u.s.tration: Water Test on Plates Painted--Except in Center Spot. Left Hand Plates Painted with Stimulative Paints, Right Hand Plates Painted with Inhibitive Paints.]

[Ill.u.s.tration: View of Steel Plates Painted with Stimulative Paints, after Immersion in Ferroxyl Jelly.]

"Three years ago the suggestion was made in a paper presented before the Tenth Annual Meeting of this Society that the various types of substances used as pigments in protective coatings might exert a stimulative or an inhibitive action on the rate and tendency to corrosion of the underlying metal. It was further suggested on a theoretical ground that slightly soluble chromates should exert a protective action when employed as pigments by maintaining the surface of the iron in a pa.s.sive condition in case water and oxygen penetrated the paint film. In view also of the well-known fact that alkalies inhibit while acids stimulate the corrosion of iron, it was suggested that the action of more or less pure pigments on iron in the presence of water should be thoroughly investigated. Two years ago this Committee invited the co-operation of Committee D-1 (then known as Committee E) in the investigation, and a special sub-committee representing the two main committees was appointed.

"The methods and results of the water-pigment tests have previously been reported and published, and need not be given in detail. Briefly, the method consisted in immersing samples of steel in water suspensions of the various pigments and blowing air through the containers for definite periods of time, the corrosion being measured by the loss in weight sustained by the test pieces. About fifty pigments which are in more or less common use for painting steel were purchased in the open market and distributed among a number of the members of the Committee, who agreed to carry out the work. Each investigator worked independently of the others, except that the same general method was followed; the time of exposure to the corroding action, however, varied in the different experiments. When the results were compared and a.n.a.lyzed by the sub-committee, it was felt that the general agreement of the results obtained by the several investigators was striking and merited further and more systematic work. As a result of these tests the sub-committee tentatively divided the pigments into inhibitors, stimulators, and indeterminates. The word 'indeterminate' was selected after considerable discussion, because the words 'neutral' or 'inert' already possess a special meaning as applied to paint technology. The Committee takes this occasion to emphatically state that in adopting this tentative cla.s.sification, the words 'inhibitive' and 'stimulative' as used by them up to the present time apply only to the results obtained in the water tests, and the inference that the results obtained have decided which cla.s.s the pigment will fall into when made into a paint with the usual vehicles and used as a protective coating on iron and steel, is not justified. In order to make this point quite clear, it has been agreed by the Committee to qualify the cla.s.sification so as to speak of the various materials tested as 'water stimulative' or 'water inhibitive.'"

[Ill.u.s.tration: Apparatus for Testing the Inhibitive Value of Pigments]

=Importance of Field Tests.= Although the laboratory accelerated tests for the determination of the relative value of structural steel paints afford information of some import, there seems to be a general opinion that the best method to follow, if information of a reliable character is to be obtained, is to make actual field exposure tests upon large surfaces. The results of the above described water-pigment tests suggested the erection of a series of steel panels on which to test out the same pigments under practical service conditions. The Paint Manufacturers' a.s.sociation of the United States erected and painted the panels, the work being under the constant supervision of the writer, and the inspection of the work under Committee U of the American Society for Testing Materials. A brief resume of the work[39] is herewith presented.

[39] Page 181, "Corrosion and Preservation of Iron and Steel"--Cushman and Gardner--McGraw-Hill Book Co., New York City.

=Pickling and Preparation of Plates.= The three types of metal[40]

selected for the test were rolled to billets, the middle of which were selected, and worked up into plates 24 inches wide, 36 inches high, and 1/8 inch in diameter--approximately 11 gauge. A number of plates of each of the metals selected, in all 450, were pickled in 10% sulphuric acid, kept at 180 to 200 degrees Fahrenheit, in order to remove the mill-scale. The plates were then washed in water, and later in 10% solution of caustic soda. Finally the plates were again washed in water and wiped dry. They were then packed in boxes containing dry lime, in order to prevent superficial corrosion. By this method the plates were secured in perfect condition, the surfaces being smooth and free from scale. Upon these pickled plates paints were applied with a definite spreading rate of 900 square feet per gallon. The unpickled plates, coated with mill-scale, were painted with the same paints, but without adopting any special spreading rate, thus following more closely the ordinary method of painting structural steel. A few extra plates of special Bessemer steel and Swedish charcoal iron were also included in the test, some of which were painted, while others were exposed without any protective coating. Plates of the three types of metal already mentioned were also exposed unpainted, both in the black and pickled condition.

[40] Bessemer Steel, Open Hearth Steel, and Pure Iron.

[Ill.u.s.tration: Front View of Steel Test Fences]

=Fence Erection and Preparation for Work.= The fences which were erected for the holding of the plates were constructed of yellow pine, the posts being set deeply in the ground and properly braced. The framework of the fence was open, with a ledge upon the lateral girders, upon which the plates might rest, and to which the plates were secured by the use of steel b.u.t.tons. After the framework had been erected, painted, and made ready for the placement of the panels, a small shed was built upon the ground, and the materials for the field test placed therein. The steel plates were unpacked from the boxes in which they were shipped, brushed off, and stacked up ready for painting. Small benches were erected, and the accessories of the work, such as cans, brushes, pots, balances, etc., were placed in position.

=Methods Followed in Painting Plates.= A frame resting upon the workbench served to hold the plates in a lateral position while being painted, room being allowed beneath the plate for the operator to place his hands in order to lift the plates from the under surface after the painting had been finished.

A pickled plate having been placed upon the framework everything was in readiness for the work. The specific gravity and weight per gallon of the paint to be applied was determined, and the amount, in grams, to be applied to each individual panel was calculated according to the following formula:

Spreading rate Sq. ft. in plate Grams paint in gal.

900 sq. ft. : 6 :: 5400 : x

The reciprocal of _x_ being the number of grams of paint to be applied to the panels.

An enamel cup was then filled with the paint and a brush well stirred within. The cup, paint, and brush were placed upon the balances and accurately weighed in grams. After most of the paint had been applied to the panel, cross-brushing of the panel was continued until the pot with brush and paint exactly counterbalanced the deducted weight. The painted panel was then set in a rack, in a horizontal position to dry.

A period of eight days elapsed between the drying of each coat. The greatest care was taken in the painting of the edges of the plates, and the racks for containing the plates after they were painted were so constructed that the paint would not be abraded while sliding the plates back and forth. The working properties of each paint, and the appearance of the surface of each plate after painting, were carefully noted and included in the report. No reductions were made to any of the paints applied except in three cases, where the viscosity was so great that it was necessary to add a small amount of pure spirits of turpentine. The amount of paint was proportionately increased in such cases, so that the evaporation of the turpentine would leave upon the plate the amount of paint originally intended.

The appearance of the completed series of test panels is shown on page 221.

=Vehicles Used and Reasons for Avoidance of j.a.pan Driers.= The pigments used were selected with the view to securing as nearly as possible purity and strength, and as already noted, were out of the same lots used in making the preliminary laboratory tests on inhibitives. They were ground in a vehicle composed of two parts of raw linseed oil and one part of pure boiled oil. Paint is generally caused to dry rapidly by the use of j.a.pan or driers. These materials contain a large amount of metallic oxides which might have some effect in either exciting or r.e.t.a.r.ding corrosion. To prevent the introduction of such a factor, these materials were not used in the test. The boiled oil, with its small percentages of metallic oxides, was sufficient, however, to cause the paints to dry in a short time after they were spread.

=Testing Effect of Various Prime Coats.= Some of the special tests made included a series of plates prime-coated with different inhibitive pigments, and these tests were designed to determine which pigments offer the best results for such work. These plates were all second-coated with the same paint. It is the opinion of the authors that any good excluding paint may be used whether it be inhibitive in action or not, provided the contact coat is inhibitive. If, however, both coats can be designed so as to have the maximum possible value from both these points of view, the best results would, of course, accrue. The only way such data can be obtained is by careful observation of the results of exposure tests.

=Combination Formulas Tested.= By selecting a series of pigments which in the water tests showed inhibitive tendencies, and properly combining these pigments into a paint, it was thought possible that a more or less inhibitive paint would be produced. If this proved to be the case, it would follow that the selection and introduction into a paint of the stimulative pigments would inevitably produce a paint unfit for use on iron or steel.

=Data on Application of Paints.= The recorded data on the application of the paint to the panels is voluminous. There is presented herewith, however, the data on two of the paints.

NO. 2, QUICK PROCESS WHITE LEAD:

Sp. Gr. of pigment 6.78 Lbs. to gallon oil 20.34 Sp. Gr. of paint as received 2.47 Wt. of paint per gallon 20.56 Grams to panel 62 Condition of paint Good Working properties Works easy Drying 24 hrs. all coats

1 coat Oct. 26 T 60 B 29.94 W. fair 2 coat Nov. 3 T 54 B 30.23 W. clear 3 coat Nov. 7 T 52 B 29.66 W. cloudy

NO. 9, ORANGE MINERAL (AMERICAN):

Sp. Gr. of pigment 8.97 Lbs. to gallon oil 26.91 Sp. Gr. of paint as received 2.97 Wt. of paint per gallon 24.74 Grams to panel 74.7 Condition of paint Good Working properties Smooth--no brush marks Drying Good

1 coat Oct. 28 T 58 B 30.01 W. cloudy 2 coat Nov. 4 T 65 B 29.61 W. cloudy 3 coat Nov. 9 T 58 B 29.91 W. clear

=Composition of Paints.= The following table gives data regarding the composition, etc., of paints applied to the steel panels.

=Results of Inspection.= The results of an inspection of the steel test plates, made by Sub-committee D representing Committee D-1 of the American Society for Testing Materials, is herewith presented: